Three-phase slurry processes, particularly those occurring in bubble columns, are widely reported in scientific literature and, hence, are well known to those skilled in the art. An example of such a three-phase slurry process is the production of hydrocarbons by means of the Fischer-Tropsch process.
Typically, a Fischer-Tropsch hydrocarbon synthesis process is conducted by contacting a stream of synthesis gas (comprising H2 and CO) with a liquid suspension of solid catalyst. The synthesis gas generally will have a H2/CO molar ratio of from 1:1 to 3:1. The dispersing liquid is primarily linear hydrocarbon reaction product. To facilitate contact between catalyst and the synthesis gas, the gas is fed into the bottom of the bubble column through a gas distributor that produces small gas bubbles.
Normally, slurry phase processes are conducted under conditions sufficient to prevent slumping of the bed, that is, under conditions that prevent catalyst particles from accumulating near the bottom of the bubble column. Thus, the settling tendency of the catalyst particles is opposed by dispersion forces created by the rising gas bubbles from the gas fed into the bottom of the bubble column.
Because it is necessary to maintain the slurry in the reactor at a constant level, liquid products are continuously or periodically removed from the reactor. In doing so, however, it is important to separate dispersed catalyst particles from the liquid being removed to maintain a constant inventory of catalyst in the reactor. Generally, the separation is conducted in a filtration zone located inside the slurry bed. The filtration zone typically comprises cylindrical filtering media through which liquid product passes from the exterior to the interior of the filtering media where it is collected and removed from the reactor. In some reactor designs, liquid product is filtered in an external filtration system.
One of the problems associated with filtration systems is the decrease in filter efficiency over time, necessitating remedial action such as backwashing the filter media, and eventually removing and cleaning the filter element or replacing it. The decrease in filter efficiency is due mainly to the presence in the liquid product of very small catalyst particles known as “fines,” which cause filters to plug. The presence of catalyst fines in the slurry liquid is due to the attrition of the catalyst that occurs over time under the turbulent hydrodynamic conditions existing in the reactor vessel.
It should also be mentioned that in external filter systems the catalyst may become deactivated because it is not always maintained at process conditions.
Thus, there is a need for increasing the effectiveness of separating catalyst particles from liquid products in a three-phase slurry process. There also is a need for controlling the fines content in a slurry reactor to prevent excessive filter backwashing and fouling. Additionally, there is a need for removing liquid product from a slurry process without the use of filters. These and other needs to are addressed by the present invention.